Organic EL televisions are being acknowledged by the general public as next generation displays after plasma televisions and liquid crystal televisions. In organic EL televisions, a light-emitting element in which a layer including an organic compound is interposed between electrodes and light is emitted by supplying a current (hereinafter referred to as a light-emitting element) is used for a pixel. Therefore, extra space or a backlight is not needed, and very thin displays can be obtained. In addition, organic EL televisions have high visibility and high response speed, and furthermore, can easily achieve a high contrast ratio. Accordingly, organic EL televisions are undoubtedly next generation displays which can display high-quality images.
However, organic EL televisions which are now on the market consume approximately 1.5 times more power than liquid crystal televisions in the same size. Organic EL televisions have the potential to achieve lower power consumption than that of liquid crystal televisions, but they have not yet been developed to that level.
These days, it is required to use fewer resources and less energy. Thus, if organic EL televisions achieve lower power consumption than that of liquid crystal televisions, the organic EL televisions can be very attractive products which can improve the quality of life and conform to environmental awareness at the same time, so a demand for lower power consumption is high.
There are many approaches to achieving lower power consumption of displays. Reduction in drive voltage of a light-emitting element itself is a very simple and effective approach. In particular, the drive voltage of a light-emitting element greatly depends on a material which is used. Hence, a material which can reduce the drive voltage of a light-emitting element is being developed.
In many cases, a light-emitting element is formed using plural layers having different functions. Typically, a layered structure in which a hole-injecting layer, a hole-transporting layer, a light-emitting layer, an electron-transporting layer, and an electron-injecting layer are provided from an anode side is used. These functional layers are each formed using a material superior in its respective function. Characteristics of the light-emitting element depend not only on characteristics of the materials used for the functional layers but also on a combination or compatibility of the materials which are used. That is, even if a material with a good property is used, its favorable feature cannot be exhibited at all with an unsuitable combination; thus, it is very important to broaden choices of materials which can be used for each functional layer.
As for materials used for forming the light-emitting element, a comparatively large number of substances are proposed for hole-transporting materials. However, under the present circumstances, there are much fewer kinds and choices for electron-transporting materials and electron-injecting materials, compared to the hole-transporting materials.
In Reference 1 (Japanese Published Patent Application No. H10-340784), an example in which an acenaphtho[1,2-b]triphenylene derivative is used for an electron injecting and transporting layer is disclosed.